This invention pertains generally to a method for producing high pressures, by converting electric potential to hydrodynamic force, that requires no moving mechanical parts and particularly to the use of electro-osmotic flow to produce a high pressure system for compressing and manipulating fluids, in general, and in packed microchannels and capillaries, in particular.
The phenomenon of electro-osmosis, in which the application of an electric potential to an electrolyte in contact with a dielectric surface produces a net force on a fluid and thus a net flow of fluid, has been known since Reuss in 1809. The physics and mathematics defining it and its associated phenomenon streaming potential, both part of a larger class of electrochemical phenomena, namely electrokinetic effects, have been extensively explored, Introduction to Electrochemistry, S. Glasstone, 1942, pp. 521-529 and R. P. Rastogi, xe2x80x9cIrreversible Thermodynamics of Electro-osmotic Flowxe2x80x9d, J. Sci. and Industrial Res., 28, 284, 1969. In like manner, electrophoresis, the movement of charged particles through a stationary medium under the influence of an electric field, has been extensively studied and employed in the separation and purification arts.
The use of electro-osmotic flow has been wide spread and has found wide ranging applications in chemical analysis. The use of electro-osmostic flow for fluid transport in packed bed capillary chromatography was first documented by Pretorius, et. al., xe2x80x9cElectro-osmosisxe2x80x94A New Concept for High-Speed Liquid Chromatographyxe2x80x9d, J. Chromatography, 99, 23-30, 1974. Although the possibility of using this phenomenon was recognized two decades ago, the effective use of this method to perform chemical analysis has only recently been demonstrated and has just begun to provide commercial utility.
Except for very general references to the fact that pressures generated by electro-osmotic flow were linearly proportional to the applied voltage (cf. Dasgupta and Liu, Analytical Chemistry, 1194, 66, 1793 and Theeuwes U.S. Pat. No. 3,923,426 at col. 1 line 23) there appeared to be no recognition in the prior art that electro-osmosis could be used to generate high pressure. Experimental studies that did explore the relationship between electro-osmosis and pressure, generally studies of streaming potential, were limited to pressures below 1 psi (Rastogi, ibid. and Cooke, J. Chem. Phys., 1995, 23, 2302). Moreover, Rastogi,. ibid., 291, has shown that the then recognized linear dependence of electro-osmotic pressure and applied electric potential begins to fail at voltages of about 300 to 400 Volts and pressures above about 0.2 to 0.3 psi and in fact, pressure begins to approach an asymptote of between 0.3 to 0.4 psi at an applied electric potential on the order of 600 Volts. Thus, prior art did not recognize and, in fact, taught away from being able to achieve pressures above about 1 psi by means of electro-osmosis. It is believed that the cause of the non-equilibrium pressure/applied electric potential effects observed in earlier work may be the result of using capillaries having too large a diameter and/or solutions having too high a conductivity which can cause undesirable heating of the electrolyte to the point where boiling and bubble formation can take place.
High-performance liquid chromatography (HPLC) is an established analytical technique that relies on high-pressure mechanical pumps (generally a gear- or cam-driven pump capable of generating pressures in excess of 5,000 psi) to drive a fluid sample through a specially prepared column. The HPLC separation medium or stationary phase is typically a thick bed packed with fine particles. Fused silica beads are often used as an HPLC column packing material. However, fused silica alone is not in general a good separation medium. Usually, a special coating is applied to the particles or the particles themselves are porous. HPLC columns can also be packed with special polymers or resins. Regardless of the column packing used, the HPLC column presents a very large resistance to flow, hence the need for high pressures to drive the sample being analyzed through the system. In HPLC the high pressure pump itself can not only comprise a substantial component of the system but also, because of the need to fill the pump as well as attendant reservoirs and plumbing, can require the use of relatively large samples. However., this proves to be particularly cumbersome in those instances where it is desired to run a second sample or samples in parallel in columns having different stationary phases because of the requirement for separate pumps for each column. Finally, there is a desire to have field-portable instruments which has been frustrated by the size, bulk, and power consumption of conventional HPLC pumps as well as the large amounts of sample required.
Conventional HPLC systems typically employ separation columns of about 3-5 mm in diameter and flow rates≈3-5 mL/min. However, miniaturization of the separation column (microbore columns) offers several advantages, including improved efficiency, mass detection sensitivity, low solvent consumption, small sample quantity, and easier coupling to detector such as mass spectrometers and flame-based detectors and several analytical methods using miniaturized or capillary columns have been developed for micro-HPLC and capillary electrochromatography (CEC). These columns generally have inside diameters of 1 mm or less.
Commercially available HPLC systems typically employ piston or cam-driven pumps to pressurize the column. However, it is difficult to adapt these pumps to provide the low flow rates under high pressure required for microbore HPLC systems. As a practical matter, cam-driven pumps with the desired stroke ratios cannot be designed for flow rates lower than about 50 xcexcl/min. A further disadvantage of cam-driven pumps is that a single pump can only provide a limited range of flow rates. This is because different flow rate ranges require cams of substantially different size and the position of the cam relative to the motor and piston is determined by the cam dimensions. Changing the positions of the motor and piston to accommodate a cam of different size is impractical because of the sensitive alignment required in piston pumps.
A alternate approach for pumping in microbore HPLC systems is the syringe-type piston pump. While this type of pump is capable of delivering solvent at a few xcexcl/min flow rate it is difficult to maintain a constant flowrate due to the continuously changing flow resistance.
What is required is a system that will provide pressure driven flows at constant and controllable flow rates, wherein the flow rate can be in the range of mL/min to xcexcl/min. Further, the system must be compatible with microbore columns and the desire for small sample quantity, low solvent consumption, improved efficiency, the ability to run samples in parallel, and field portability.
The present novel invention uses electro-osmotic flow to provide a high pressure. hydraulic system, having no moving mechanical parts, for pumping and/or compressing fluids, for providing valve means and means for opening and closing valves, for controlling fluid flow rate, and manipulating fluid flow generally and in capillary-based systems (microsystems), in particular. Moreover, because of the compact nature of the inventive high pressure hydraulic pump it is now possible to construct a conventional or capillary-based HPLC system that fulfills the desire for small sample quantity, low solvent consumption, improved efficiency, the ability to run samples in parallel, and field portability. Control of pressure and solvent flow rate is achieved by controlling the voltage applied to an electrokinetic pump.
Contrary to prior art teachings, the inventors have discovered by both theoretical prediction and experimental studies that in a capillary-based system electro-osmotic flow can generate pressures as high as 5,000 psi and that the relationship between electro-osmotic pressure and applied electric potential is linear up to and including pressures as high as 5,000 psi (FIG. 1).
The ability to pressurize a fluid by means of an electric potential provides a means for imparting net power to the fluid and by this means to transmit and use this net power to perform work (apply force) on some system. It will be appreciated that the ability to convert the hydraulic action produced by the system disclosed herein to mechanical action and work can encompass exerting hydraulic pressure on a diaphragm or hydraulic drive of a positive displacement fluid motor, or hydraulic flexure of a fluid-filled member, or expansion or contraction of a fluid-filled bellows, or extension or retraction of a fluid-filled piston, or any other means known in the art of converting hydraulic action, power and work to mechanical action, power and work.
The inventive high pressure hydraulic pump, hereinafter known as an electrokinetic pump or EKP, comprises at least one tube or channel, that can be a capillary channel or microchannel, forming a fluid passageway containing an electrolyte and having a porous dielectric medium disposed therein between one or more spaced electrodes. The porous dielectric medium can include small particles, high surface area structures fabricated within the microchannel, or microporous materials. An electric potential is applied between the electrodes that are in contact with the electrolyte, that can be aqueous or an organic liquid or mixtures thereof, to cause the electrolyte to move in the microchannel by electro-osmotic flow. The electric field applied across the EKP by the electrodes will cause the electrolyte contained in the porous dielectric medium to flow and presented with an external flow resistance will create a pressure at the down stream end of the EKP. The flowrate of the electrolyte is proportional to the magnitude of the applied electric field (V/m applied across the EKP) and the pressure generated is proportional to the voltage across the device. The direction of flow of the electrolyte is determined by both the nature of the electrochemical interaction between the porous dielectric medium and the electrolyte and the polarity of the applied electric potential. In contrast to prior art hydraulic pumps, and HPLC pumps in particular, the present invention can be realized by integrating part or all of the described components on a chip or micro-scale device, i.e., a device wherein the components have dimensions less than about 0.1 mm. Thus, the EKP is a compact and efficient device which converts electric power to hydraulic power in the working fluid and has been shown to be capable of generating hydraulic pressures greater than 5000 psi. The EKP has been fully described in prior co-pending U.S. patent application Ser. No. 08/882,725 entitled xe2x80x9cElectrokinetic High Pressure Hydraulic Systemxe2x80x9d filed Jun. 25, 1997, incorporated herein in its entirety.
The above-described electrokinetic high pressure hydraulic system has several advantageous features. There are no moving mechanical parts and all liquid seals, thus the system is not subject to frictional wear. Since the system is driven electrically and has no moving mechanical parts it can be rapidly turned on and off. By applying periodic electrical potentials, whose periods can be various functions of time, to a plurality of spaced electrodes different timing arrangements such as might be useful for varying compression and valving cycles can be effected. Because a porous dielectric medium with a pore size of between about 2-200 nm presents a very high resistance to pressure driven flow, the high-pressure hydraulic system disclosed here acts as a form of check valve when, having achieved a high pressure, the electric potential is switched off. Moreover, the system is capable of remote operation.
In one aspect, the invention is configured such that an electrolyte contained in a porous dielectric medium disposed within a capillary or microchannel can act as a valve; the electrolyte being selectively moveable between a first position opening communication between a fluid inlet and an outlet and a second position closing communication between the fluid inlet and outlet. Opening and closing the valve is provided by applying an electric potential between the spaced electrodes sufficient to cause the electrolyte to move from the first position to the second. The process can be reversed simply by reversing the polarity of the electric potential. By providing an actuator at the fluid outlet it is possible to transfer the hydraulic force generated by the invention into rotary or rectilinear motion. Furthermore, it is contemplated that the hydraulic force developed at an actuator can be transferred to more than one hydraulic line.
In a second aspect, the invention is configured to compress a fluid, which can be either a liquid or a gas. Here a fluid outlet can be either completely sealed or constricted such that when an electric potential is applied between spaced electrodes, movement of the electrolyte causes the fluid which resides between the electrolyte and the sealed or constricted outlet to be compressed.
In a third aspect of the present invention, the EKP and an HPLC column are connected such that the pressure developed by the EKP is applied to the inlet of an HPLC column. The HPLC can be connected directly to the EKP or connection can be made indirectly such that the force developed by the EKP is applied to the HPLC column through a secondary means, such as a positive displacement pump that can be a bellows, diaphragm or piston pump.
It will be appreciated by those skilled in the art that it is desirable to eliminate the generation of any gases that could arise as a consequence of electrolysis of the EKP electrolyte. This can be accomplished by several means known to the art. By way of example, a section of ultra micro-porous material, such as the porous glass sold under the trademark VYCOR, having nominally 4 nm pores, or a membrane such as that sold under the trademark NAFION saturated with electrolyte can be interposed between the electrode providing connection to the high pressure fluid junction and the junction itself. The ultra micro-porous material carries the current but the pores are sufficiently fine that pressure-driven or electro-osmotic flow is negligible.